This disclosure pertains to microlithography of a pattern, defined by a reticle or mask (generally termed xe2x80x9creticlexe2x80x9d herein) from the reticle to a sensitive substrate (e.g., a semiconductor wafer coated with a substance, termed a xe2x80x9cresist,xe2x80x9d that is imprintable with an image of the pattern) using an energy beam. Microlithography is a key technique used in the manufacture of microelectronic devices such as integrated circuits, displays, magnetic pickup heads, and micromachines. More specifically, the disclosure pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as the energy beam. Even more specifically, the disclosure pertains, in the context of charged-particle-beam microlithography, to obtaining a desired exposure dose on the substrate regardless of the thickness of the membrane portion of the reticle, thereby allowing throughput to be increased.
Charged-particle-beam (CPB) microlithography offers prospects of substantially higher pattern-transfer resolution than currently achievable using optical microlithography. Unfortunately, throughput achievable with CPB microlithography still is substantially less than the throughput achievable with optical microlithography. (xe2x80x9cThroughputxe2x80x9d denotes the number of production units such as wafers that can be processed per unit time using the subject technology.) Substantial development efforts in CPB microlithography have been directed to improving throughput.
The main reason for low throughput with CPB microlithography is the current impossibility of fabricating a reticle defining the entire pattern in a manner allowing the entire pattern to be exposed in a single exposure xe2x80x9cshot.xe2x80x9d Another reason is the current impossibility of providing illumination and projection xe2x80x9copticsxe2x80x9d capable of projecting an entire pattern in a single shot while also providing satisfactorily low aberrations in the projected image.
One current example of a CPB microlithography technology that has been put to practical use is xe2x80x9ccell projection,xe2x80x9d which involves the repeated transfer and exposure of a limited number of types of graphic portions of a device pattern (hence, this technique also is termed xe2x80x9cgraphic-portion batch exposurexe2x80x9d). The graphic portions are defined as respective apertures in an aperture plate that are exposed in a mix-and-match manner so as to reconstruct the pattern portion-by-portion on the substrate. Unfortunately, the maximal throughput obtainable using graphic-portion batch exposure is about one order of magnitude too low for practical application in actual mass-production of semiconductor chips.
To improve throughput over that obtainable using cell projection, so-called xe2x80x9cdivided reticlexe2x80x9d reduction projection-exposure apparatus have been developed for CPB microlithography. In the divided-reticle technique, the entire chip pattern as defined on the reticle is divided into multiple xe2x80x9csubfieldsxe2x80x9d each defining a respective portion of the overall pattern. The apparatus directs a charged-particle illumination beam to illuminate a selected subfield on the reticle. Portions of the illumination beam passing through the illuminated portion of the reticle are projected by a projection-optical system onto a corresponding selected region on the substrate. The subfields are projected according to a predetermined order, and are imaged such that the images of the subfields are placed contiguously with each other (i.e., xe2x80x9cstitchedxe2x80x9d together) on the substrate. Projection of the images normally is performed with xe2x80x9creductionxe2x80x9d (demagnification); hence, the image of a subfield on the substrate is smaller (usually by an integer factor such as 4 or 5) than the corresponding subfield on the reticle. Further details of this technique can be found in U.S. Pat. No. 5,260,151, incorporated herein by reference, and in Japan Kxc3x4kai (Laid-Open) Patent Document No. Hei 8-186070.
In divided-reticle CPB microlithography, generally two types of reticles are used: non-stenciled-membrane reticles and stenciled-membrane reticles, as shown in FIGS. 7(A) and 7(B), respectively. Both types of reticles are fabricated from a semiconductor wafer (typically silicon). The reticles are shown in the figures as respective elevational sections of a portion of each. The non-stenciled-membrane reticle also is termed a xe2x80x9cscattering-membranexe2x80x9d reticle, and the stenciled-membrane reticle also is termed a xe2x80x9cscattering-stencilxe2x80x9d reticle.
The non-stenciled-membrane reticle 10-1 shown in FIG. 7(A) comprises, for example, a SiNx (silicon nitride) layer 62 formed on the xe2x80x9clowerxe2x80x9d (downstream-facing) surface of a Si (silicon) substrate 61. The SiNx layer 62 is relatively non-scattering to charged particles of an illumination beam EB incident from an upstream direction. I.e., incident charged particles pass through the SiNx layer 62 with little to no forward scattering. On the xe2x80x9clowerxe2x80x9d surface of the SiNx layer 62 is a W (tungsten) layer 63. The W layer 63 is highly scattering to charged particles of the incident illumination beam EB. I.e., incident charged particles pass through the W layer 63 with substantial forward scattering. After forming the layers 62, 63 the Si substrate 61 is removed by anisotropic dry-etching or the like from regions corresponding to subfields 64, thereby forming in the subfields 64 respective self-supporting membrane portions 65 composed of the SiNx layer 62 and the W layer 63. In the subfields 64, portions of the W layer 63 are selectively removed to form non-scattering regions 66 and scattering regions 67. In each subfield 64, the non-scattering regions 66 and scattering regions 67 collectively define, on the membrane 65, a respective portion of the chip pattern to be projected onto a sensitive substrate (not shown).
Whenever the non-stenciled-membrane reticle 10-1 is irradiated with a CPB illumination beam (e.g., electron beam EB), the non-scattering regions 66 transmit respective portions of the beam, with substantially no scattering, to the sensitive substrate. The scattering portions 67, on the other hand, greatly scatter respective portions of the incident beam. The scattered particles propagating downstream of the reticle are absorbed and thus blocked by a contrast aperture or the like (not shown, but see FIG. 1) to prevent them from reaching the sensitive substrate. The non-scattered particles pass through the contrast aperture to the sensitive substrate. Thus, an image of the pattern can be formed with satisfactory contrast on the sensitive substrate.
Referring now to the stenciled-membrane reticle 10-2 shown in FIG. 7(B), the silicon substrate 71 is removed by anisotropic dry-etching or the like from regions corresponding to subfields 74, thereby forming in the subfields 74 respective self-supporting membrane portions 75 composed of residual silicon substrate 71. Portions of the membranes 75 are selectively removed to form beam-transmissive apertures 76. The apertures 76 and remaining portions 77 of the membranes 75 collectively define a pattern to be projected onto a sensitive substrate (not shown).
Whenever the stenciled-membrane reticle 10-2 is irradiated with a CPB illumination beam (e.g., electron beam EB), the apertures 76 transmit respective portions of the beam, with substantially no scattering, to the sensitive substrate. The membrane portions 77, on the other hand, greatly scatter respective portions of the incident beam. The scattered particles propagating downstream of the reticle are absorbed by a contrast aperture or the like to prevent them from reaching the sensitive substrate. The non-scattered particles pass through the contrast aperture. Thus, an image of the pattern can be formed with satisfactory contrast on the sensitive substrate.
The transmissivity of a CPB-transmitting portion of a non-stenciled-membrane reticle used in an electron-beam microlithography apparatus is a function of the thickness of the CPB-transmissive layer, as discussed in J. Vac. Sci. Technol. B16:3385, 1998. FIG. 8 is a representative plot of the transmissivity of the CPB-transmitting portion of a non-stenciled-membrane mask, as a function of SiNx membrane thickness (i.e., thickness of the CPB-transmissive portion of the reticle). The plotted data were obtained using an electron beam subjected to an acceleration voltage of 100 keV and a contrast aperture angle of 0.5 mrad. In FIG. 8, the transmissivity is about 40% whenever the SiNx film thickness is 50 nm, and about 15% whenever the SiNx film thickness is 100 nm. Thus, because the transmissivity varies with thickness of the SiNx film, errors in the SiNx film thickness can result in substantial variations in exposure-beam current reaching the sensitive substrate, producing substantial errors in exposure.
In a conventional reticle-production process, the SiNx film is formed by chemical vapor deposition (CVD) on a reticle substrate. Even though CVD can form a substantially uniformly thick layer of SiNx on a given reticle substrate, it is exceedingly difficult to form this film at a uniform thickness from one reticle substrate to another. This difficulty arises from inherent limitations in the accuracy and precision of layer-thickness control achieved in CVD. Since the transmissivity of the CPB-transmitting portion of the non-stenciled reticle used in divided-reticle CPB microlithography varies with the thickness of the SiNx film, substantial changes arise in exposure dose whenever the reticle is changed.
Whereas, in non-stenciled-membrane reticles as mentioned above, the transmissivity of the CPB-transmitting portion is a function of the thickness of the SiNx layer, the transmissivity of the CPB-transmitting portion of a stenciled-membrane is always 100%. Thus, depending on the type of reticle being used, a greater than two-fold difference in the transmissivity of the CPB-transmitting portion could be experienced. Whenever both non-stenciled-membrane reticles and stenciled-membrane reticles are used in the same microlithographic exposure apparatus, a pattern defined on a non-stenciled-membrane reticle with low transmissivity requires an exposure time that is more than twice the exposure time of a pattern defined on a stenciled-membrane reticle. As a result, substantially decreased throughput is achieved when using the non-stenciled-membrane reticle compared to using the stenciled-membrane reticle.
In view of the shortcomings of conventional apparatus and methods as described above, the present invention provides, inter alia, charged-particle-beam (CPB) microlithography methods and apparatus with which an accurate exposure dose can be obtained regardless of the transmissivity of the membrane of a non-stenciled-membrane reticle being used. Thus, exposure-dose variations conventionally experienced whenever the reticle is changed are prevented, and throughput correspondingly increased.
To such end and according to a first aspect of the invention, charged-particle-beam (CPB) microlithographic exposure methods are provided. In an embodiment of such a method a reticle is placed relative to a charged-particle illumination beam. The reticle defines a pattern to be transferred onto a sensitive substrate and comprises a CPB-transmissive membrane layer and a CPB-scattering layer selectively formed on the CPB-transmissive membrane layer. An illumination beam passes through the CPB-scattering layer with substantial scattering of charged particles of the beam, and passes through the CPB-transmissive membrane layer with relatively little scattering. The CPB-scattering layer and CPB-transmissive membrane layer collectively define elements of the pattern on the reticle, and the charged particles of the beam propagating downstream of the reticle collectively define a patterned beam. Propagation of at least most of the charged particles of the patterned beam that were scattered by passage through the CPB-scattering layer are blocked so as to prevent the blocked charged particles from propagating to the substrate, while the relatively non-scattered charged particles of the patterned beam are projected and imaged onto the substrate. The illumination beam is irradiated on a measurement region of the reticle at which only the CPB-transmissive membrane layer is present, and a beam current of the resulting patterned beam propagating downstream of the region to the substrate is measured. From a pre-set target exposure dose for the substrate and from the measured beam current, an actual exposure condition for the reticle is determined.
In the exposure method summarized above, the reticle including a CPB-transmissive membrane layer and a CPB-scattering layer is termed a xe2x80x9cnon-stenciled-membrane reticle.xe2x80x9d With such a reticle the exposure dose at the substrate is accurately and precisely controlled regardless of the thickness (and thus regardless of the CPB transmissivity) of the non-stenciled-membrane reticle. This method also allows a less strict thickness tolerance of the CPB-transmissive membrane layer, which reduces the cost of the reticle.
The illumination beam typically is generated by a CPB source and passed through an illumination-optical system to the reticle. If the measured beam current deviates significantly from a previously specified beam current, then at least one of the CPB source and illumination-optical system can be adjusted so as to bring the beam current on the substrate within a desired range including the previously specified beam current.
According to another embodiment, an electron-beam microlithographic exposure method is performed using an electron-beam microlithographic exposure device comprising an illumination-beam source, an illumination-optical system through which the illumination beam passes, a reticle stage, a projection-optical system through which a patterned beam passes, and a substrate stage including a beam-current sensor. In the method a reticle is mounted on the reticle stage relative to the illumination beam. The reticle defines electron-transmitting (and non-forward-scattering) regions and electron-forward-scattering regions that collectively define a pattern on the reticle, and a transmitted-current-detection window that is as transmissive and non-forward scattering to the illumination beam as any of the electron-transmitting regions of the reticle. The substrate stage is moved to place the beam-current sensor in a beam-current-sensing position, and the reticle stage is moved to place the transmitted-current-detection window at an illumination-beam-irradiation position. The transmitted-current-detection window is irradiated with the illumination beam while a beam current of the resulting patterned beam is measured using the beam-current sensor. A determination is made of whether the measured beam current is outside a predetermined beam-current range defined between lower and upper thresholds. If the measured beam current is outside the predetermined range, then at least one parametric setting of at least one of the illumination-beam source and illumination-optical system is changed so as to bring the measured beam current within the predetermined range. From the measured beam current within the predetermined range, calculations are made of a beam-current density at the substrate stage and an exposure time for exposing the substrate with a specified dose using the reticle.
The method summarized above can include the step of changing the exposure time depending upon whether the reticle is a non-stenciled-membrane reticle or a stenciled-membrane reticle.
Another aspect of the invention is directed, in the context of a CPB microlithographic exposure method in which a pattern defined by a reticle is transferred from the reticle to a sensitive substrate, to methods for controlling exposure at the substrate. In an embodiment of such a method, a reticle is placed relative to a charged-particle illumination beam produced by a CPB source and passing through an illumination-optical system. The reticle defines a pattern to be transferred onto a sensitive substrate. Depending upon whether the reticle is a stenciled-membrane reticle or a non-stenciled-membrane reticle, one or both the CPB source and illumination-optical system is adjusted accordingly to achieve a desired exposure at the sensitive substrate. Thus, a situation is prevented in which the required exposure time differs greatly whenever both non-stenciled-membrane reticles and stenciled-membrane reticles are used on the same CPB microlithography apparatus.
The method summarized above can include the step, after at least one of the CPB source and illumination-optical system has been adjusted according to the type of reticle, of measuring a CPB beam current as incident on the sensitive substrate. In this instance, exposure conditions for the substrate can be determined from the measured CPB beam current and from a preset exposure dose.
According to another aspect of the invention, CPB microlithographic exposure apparatus are provided. An embodiment of such an apparatus comprises a CPB source that emits an illumination beam toward a reticle defining a pattern to be transferred to a sensitive substrate. The apparatus also includes a reticle stage, an illumination-optical system, a projection-optical system, and a substrate stage. The reticle is mounted on the reticle stage. The illumination-optical system is situated between the CPB source and the reticle stage and is configured to irradiate a region of the reticle with the illumination beam. The projection-optical system is situated downstream of the reticle stage and is configured to project and image a patterned beam, propagating downstream of the reticle and comprising scattered and non-scattered charged particles of the illumination beam passing through the illuminated region of the reticle, onto the sensitive substrate. The substrate stage is situated downstream of the projection-optical system and comprises a beam-current sensor. A sensitive substrate is placed on the substrate stage for exposure. The apparatus also includes a controller that is connected to and configured to control respective operations of the CPB source, the illumination-optical system, the reticle stage, the projection-optical system, the substrate stage, and the beam-current sensor. A beam current of the patterned beam reaching a position on the sensitive substrate is measured by the beam-current sensor. The controller calculates exposure conditions for the sensitive substrate based on which of a plurality of types of reticles is mounted to the reticle stage and based on the beam current sensed by the beam-current sensor.
In the apparatus summarized above, whenever a reticle is mounted to the reticle stage, the controller adjusts a parameter of at least one of the CPB source and illumination-optical system based on the type of reticle mounted to the reticle stage. For example, the reticle can be a stenciled-membrane reticle or a non-stenciled-membrane reticle.
According to yet another aspect of the invention, reticles are provided for use in CPB microlithography. An embodiment of such a reticle comprises a CPB-transmissive membrane layer and a CPB-scattering layer selectively formed on the CPB-transmissive layer. A charged particle beam can pass through the CPB-scattering layer with substantial forward scattering of charged particles of the beam and can pass through the CPB-transmissive membrane layer with relatively little forward scattering. The CPB-scattering layer and CPB-transmissive layer collectively define elements of a pattern on the reticle. The reticle also includes a transmitted-current-detection window spanned by the CPB-transmissive membrane layer but not by the CPB-scattering layer. The transmitted-current-detection window is situated in a region of the reticle that does not define any pattern elements.
The reticle can comprise a pattern-defining region and a non-pattern-defining region. In such a configuration the pattern elements are defined in the pattern-defining region. The pattern-defining region is divided into multiple subfields each defining a respective group of the pattern elements. The transmitted-current-detection window is located in the non-pattern-defining region. The pattern-defining region typically includes support struts separating the subfields from one another.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.